Surface modification for substrate materials is an available approach which can not only endow new properties through forming functional surfaces but also maintain the inherent performance. In other words, the advantages of various materials can be combined together by surface modification for the needs in production and life. The usually used surface modification methods can be summarized into three categories: chemical grafting, e.g., atom-transfer radical-reaction, interface cross-linking, element substitution; physical compounding, e.g., particle doping, surface coating, blending; physicochemical grafting, e.g., radiation grafting, plasma treatment, photo-initiated grafting. Hereinto, chemical grafting methods are achieved with the reaction between the modification reagent and the substrate materials, which would change the molecule structure of the substrate materials, and consequently impair the inherent performance to a certain extent, e.g., mechanical strength and chemical durability, etc. Physical compounding methods are based on the non-covalent interaction between the substrate materials and the surface modification layer. In the case that the substrate materials and the modification materials are significantly differing from each other in properties, they can hardly combine together with high strength, so that the functional surface layer is easy to be damaged in use, and resulting in regressive performance and short service life.
Surface modification based on dopamine polymerization is a newly developed but highly efficient technology inspired by mussel adhesion. It is a new direction for fabricating high performance composite materials, and the foundation is the oxidative auto-polymerization of dopamine and/or its derivatives on substrate surface. Since the first report in 2007, surface modification based on dopamine polymerization has been widely attempted in many fields, e.g., biological imaging, probing for drug tracing, photo-catalysis and water processing. Polydopamine modification has two advantages: 1) The modification is carried through merely with the oxidative auto-polymerization of coating monomers, and the molecular structure of substrate materials is unchanged, so that the advantages of various materials could be combined better; 2) The coating monomers, i.e., dopamine and/or its derivatives, are amphiphilic materials, which can be adsorbed intensively on most of inert materials and have good adaptability. Much more, the coating layer formed by dopamine (and/or its derivatives) polymerization has abundant active groups, e.g., amino, imino and phenolic hydroxyl groups, which can be employed as the intermediate layer for further modification and then combine the advantages of different materials difficult to be combination.
After a long-term investigation, the limitations of surface modification based on dopamine polymerization have been recognized. The stability of the polydopamine in organic solvents (or aqueous solutions containing organic solvents) and acidic/alkalic environments is poor, which is an bottleneck restricting polydopamine modification in practical application. This instability can be attributed to the low-molecular-weight oligomers of dopamine, which are variable in solubility in the polymerization process and could deposit on the substrate materials more or less along with the precipitation of high-molecular-weight polydopamine for modification. Under the environment of organic solvent or acidic/alkalic solution, the dopamine oligomers could be dissolved gradually in the long-term utilization, which would seriously impair the dense degree and the continuity of the polydopamine coating layer for surface modification, and thereby affect the long-term stability, limit the application scope.
Given all that, with the intent to further expand the application scope of surface modification technology based on dopamine polymerization, it is necessary to enhance the long-term stability of polydopamine coating layer in organic solvents (or aqueous solutions containing organic solvents) and acidic/alkalic environments. Accordingly, the procedure about cross-linking solidification is proposed in the present invention to convert the soluble low-molecular-weight dopamine oligomers into their insoluble three-dimensional networks, which can greatly reduce the dissolution of coating layer in organic solvents and acidic/alkalic environments, and thereby enhance the stability and the service life of polydopamine coating layer for surface modification. Such a procedure is established reliably on the basis including the polymerization principle of dopamine, the formation process of coating layer, as well as the failure mechanism of polydopamine coating in severe environments. In order to realize the cross-linking solidification, the crucial matter is to screen an eligible cross-linking agent which can react with the active functional groups of polydopamine.